The fall of Yabroud would deal a signifcant blow to rebels since the initiative passed to the government in spring of 2013. Photograph: Sana/Reuters

Syria claims its military has seized a key town on the Lebanese border that was the target of a months-long offensive. Activists said fighting was continuing but the government was in control of much of Yabroud.

Yabroud was a key supply line for rebels into neighbouring Lebanon and overlooked an important cross-country highway. Its fall, coming as the Syrian conflict enters its fourth year, would be a significant blow to rebels since the initiative passed to the government in the spring of 2013.

It is the last major rebel-held town in the mountainous Qalamoun region, where President Bashar al-Assad's forces have been waging an offensive for months to try to cut rebel supply lines across the porous border into eastern Lebanon. Its fall would come a week after the Syrian army seized the village of Zara, which also served as a conduit for rebels from northern Lebanon into central Syria.

Syria's state news agency, Sana, reported that military forces seized Yabroud early on Sunday and were combing the city to remove booby-traps and bombs and hunt down rebel holdouts.

Kasem Alzein, a Syrian pro-rebel doctor who lives in the nearby border town of Arsal, said military forces entered the eastern part of Yabroud and that rebels fled to the nearby town of Flita. He said a small hardcore group of fighters said they would fight to the death in the city.

"They don't want to surrender," he said, adding that supplies were cut off and weapons promised to rebels never arrived.

"Qusair will repeat itself," Alzein said, referring to the strategic rebel-held town on the Syrian border that fell to pro-Assad forces last summer. As in the Qalamoun offensive, Lebanese Hezbollah militants played a key role backing government troops.

Gunfire could be heard on footage broadcast live by the Lebanon-based TV station al-Mayadeen, which also showed troops walking through empty streets.

Meanwhile, a flare-up of violence in the northern Lebanese city of Tripolihas left 12 people dead in recent days, Lebanon's state-run news agency said. NNA said the latest fatality was a soldier who was killed on Sunday when attackers fired on his armoured vehicle with rocket-propelled grenades and small arms. His death brings the death toll to 12 since clashes erupted Thursday, the NNA said.

The clashes pit Sunni gunmen from Bab Tabbaneh who back Syria's Sunni-majority rebels against rivals from nearby Jabal Mohsen, dominated by the Alawite sect, Assad's faith.

Sunni gunmen have also attacked Lebanese soldiers, accusing them of loyalty to rival sectarian factions in Lebanon.